1. Field of the Invention
The present invention relates to an image forming apparatus, and specifically relates to an apparatus for reproducing the image of a document disposed on a document platen on a sheet supported on a transfer drum.
2. Description of the Related Art
FIG. 17A shows the copy process of a conventional color copying apparatus.
In this copying apparatus, a single sheet P1 is supported on transfer drum 202, and a single sheet color document is disposed on the document platen. A copying operation for two document sheets in this copying apparatus is accomplished by the following sequence.
The document image is first scanned for cyan color and a corresponding electrostatic latent image is formed on the surface of a photosensitive member not shown in the drawing, said latent image is developed by cyan toner, and transferred to sheet P1. Then, scanning, latent image formation, and development are accomplished for magenta color, and subsequently scanning, latent image formation, and development are accomplished for yellow color, whereupon, finally, scanning, latent image formation, and development are accomplished for black color. Thereafter, the document disposed on the document platen is exchanged, and a total of four scanning, latent image formation, and developing operations similar to those described above are accomplished for the exchanged document. Thus, the copying operation of two color document sheets is completed.
EP 411918 discloses an apparatus provided with a function for discriminating color document regions and black and white document regions of the various divisional regions of document disposed on a document platen. In this apparatus, when a book-type document is disposed on the document platen, each page of the open-faced book is discriminated as to whether or not it is a color page or a black and white page. When a color page is discriminated, color copying is executed, whereas when a black and white is discriminated, black and white copying is executed so as to prevent wasteful color copying of black and white pages.
U.S. Pat. No. 4,712,906 discloses an apparatus for forming electrostatic latent images corresponding to image information of two images in a row on the surface of a photosensitive member, which are sequentially developed with toner and subsequently sequentially transferred onto two sheets supported on a transfer drum.
The method shown in FIG. 17A is disadvantageous insofar as a long time is required until completion of the because time is required to scan each document separately as well as to exchange documents.
Although time for exchanging two document sheets is not required in the book-type document mode (a mode wherein two document sheets are presumed to be two pages of an open book) of the apparatus disclosed in EP 411918, scanning time is required to separately scan each page with the result that a long time is required to complete the copying operation.
The apparatus disclosed in U.S. Pat. No. 4,712,906 is constructed such that image information of two images is read out from an image memory, therefore requiring a large-capacity memory, and increasing the cost of the apparatus.